The resonance tube of the sound generator is frequently adapted to be connected to a furnace or a drying plant, and the sound waves - often of a low frequency, i.e. a frequency less than about 50 Hz--are used for keeping heat exchange surfaces of the said furnace or drying device free from deposits of soot or material.
As an alternative to the method of providing gas pulses by a motor powered, oscillating piston it is possible to provide the pulses by a valve governed compressed air source. However this method is more expensive in operation.
In some furnaces or drying plants a finely divided material is formed and is liable to enter into the resonance tube and to pass between the piston and its surrounding cylinder wall. This finely divided material may cause wear on the piston rings, the piston and the cylinder wall. It may even pass into the crank casing containing the connecting rod and into the motor and the motor bearings. In plants or devices in which such materials are present or formed it may be necessary to use the more expensive alternative in which the gas pulses are provided by a valve governed supply of compressed air. However, in many cases even this solution is impossible. E. g. in case the abrasive, finely divided material is a nutrient which should not be mingled with compressed air in order to avoid impurities or oxidation.
Leakage between a piston and its surrounding cylinder wall may be avoided by using a membrane piston rigidly clamped at its periphery to a wall of a chamber surrounding the piston. In case of producing a low frequency sound a sinus shaped pressure pulse is provided and passed into the resonance tube. The pressure variations will cause a conventional membrane to flutter between end positions. As a consequence of variations in bending stress it will detonate and crack only after a few million piston strokes which is far from being acceptable.